A horizontal packaging machine known in prior art comprises a supporting frame that includes a protective covering enclosing a packaging unit, a handling device for transporting the products to the end of the frame, a conveyor unit for feeding the products from the handling device to the packaging unit, and an outfeed unit for withdrawing the packaged products.
The packaging unit comprises a roller having rolled around it a film of synthetic plastic material, usually polyethylene based, that is unwound by a suitable unreeling device and wound around a shaping device or tunnel where it is sealed continuously.
The products to be packaged are fed into the shaping device or tunnel, either continuously or intermittently, and, at the same time, the sealed film is pulled off the shaping device.
At the outfeed end of the shaping tunnel, there is a sealing and cutting device which seals the film transversely to form the bottom of the package and then cuts the package off the remaining film which is still wound around the shaping device.
As the film is unwound from the roller and wound around the shaping tunnel, its two longitudinal edges are placed side by side and sealed together by a pair of heated jaws located at the shaping tunnel.
The packaging machine features electronic control circuitry for synchronising the film sealing speed with the feed speed so as to prevent the film from being pulled and stretched out of shape at the seal.
Advantageously, the packaging machine is equipped with a cutter that separates the packages from each other in such a way that the transversal seal forming the bottom of one package normally coincides with the top of the preceding package.
This is done with a quick and sure action, as described in European patent application No. EP-A-1106506.
In this patent application, the film is unwound by the combined action of an unreeling unit and two pairs of feed rollers, one downstream and one upstream of the sealing and cutting device, while a pair of tapes form a zip closure which will allow the package to be re-used after opening.
The two tapes are placed over the film just upstream of the first pair of feed rollers so that the sealing and cutting device makes a package whose zip closure is guaranteed by the adhesion of the two tapes themselves.
One drawback is that if the package is sealed continuously, the cut made to separate one package from the next may break the seal, thus causing irremediable damage to the product inside.
With regard to this problem, the aforementioned patent application provides for electronic control of the speeds of the unreeling unit and of the pair of feed rollers. Slowing and accelerating the two speeds at the film portions to be cut creates a certain discontinuity in the pair of tapes forming the zip closure, resulting in an imperfect seal.
This patent, therefore, although it overcomes some problems, is not free of disadvantages.
One disadvantage is that the use of electronic devices to control speed occupy more space and make maintenance and set up more onerous.
Another disadvantage is that the provision of numerous sources of motion, such as unreeling units and pairs of feed rollers, upstream of the sealing and cutting unit leads to frequent crumpling of the film and tapes, which in turn means extended machine downtime to rearrange the film.